The present invention relates to a negative-angle press-working die for forming an article having a complicated curved shape, such as a side panel of an automotive body, by pressing a plate. More specifically, the present invention relates to a negative-angle press-working die including a top die and a bottom die for forming an article having a negative-angle part which protrudes into the bottom die beyond the path of the top die when a plate is placed on the bottom die and is pressed by advancing the top die into the cavity of the bottom die.
A conventional press-forming method of forming an article having a negative-angle part forms the article by several working cycles. The press-forming method forms parts of the article excluding the negative-angle part by a first working cycle, and then forms the negative-angle part by a second working cycle.
For example, as mentioned in JP-A No. 2001-347319 or JP-A No. 2001-47135, a semifinished article not having any negative part is formed by holding a plate between a holding die held on a top die, a support base held on a bottom die, and an end member holding part of a swing die, a negative-angle part is formed by a slide cam member which moves vertically as the holding die of the top die moves vertically, the swing die is turned to complete the press work, and the plate is extracted.
A press-forming die proposed in JP-B No. 8-256 has a plurality of forming parts provided with a plurality of cylindrical, rotary cams each provided with an axial groove and having a negative-angle part formed at the edge of the axial groove and a length determined so that the negative-angle part is formed in the rotary cam.
Since the swing die is provided with a rotating support shaft extended parallel to the bottom surface of the body of a press-forming machine, parts of the parting line between a fixed support base and the support part of the swing die in different sections have different heights, respectively, when the swing die is used for forming an article having a curved shape curving in a big height range between parallel planes spaced a long distance apart, such as a front pillar of an automobile. Therefore, even if the fixed support base can be designed for one of the sections, the fixed support base cannot be formed for other sections. In addition, it is difficult to form a fixed support base for forming such an article. Thus, the press-forming process is divided into a plurality of steps or the plate is supported only by the workpiece support part of the rocking member without using the fixed support base.
Since the press-forming process can be performed only for a narrow range, press-forming dies are used, for example, to form an automotive panel including a front pillar, a roof, a middle part and a rear part. Thus, it is inevitably necessary to repeat the press-forming process many times.
A lift pin for tilting the swing die has a spherical tip that comes into point-contact with the bottom of the swing die, and slides along the bottom of the swing die to lift up the L-shaped rail end of the swing die. Therefore, energy is wasted by friction and serious scratching is possible.
In the press-forming die employing the cylindrical rotary cams having the axial groove and the negative-angle part formed at the edge of the axial groove, the rotary cams are rotational shafts. Therefore, the press-forming die has problems with machining accuracy due to the deconcentration of the axes and with sliding friction due to the sliding of the entire circumferences.
In the composite press-forming die formed by arranging the plurality of forming parts including the plurality of cylindrical rotary cams, rotational interference between the adjacent rotary cams is prevented by adjusting the rotating speed by a timing plate. Such adjustment is unable to cope with press work for forming an article having a curved surface curving in a big height range, needs a complicated mechanism, and has problems with reliability and cost.